The present invention relates to novel aspartates, a process for preparing them from primary amines and maleates and to their use as reactive components for polyisocyanates in two-component polyurethane coating compositions and for preparing polyurethane prepolymers.
Two-component coating compositions which contain, as binder, a polyisocyanate component combined with one or more isocyanate-reactive components are known. They are suitable for preparing high quality coatings which are hard, elastic, abrasion resistant, solvent resistant and weather resistant.
Secondary polyamines which contain ester groups have become established in the two-component surface coating industry. They are particularly suitable, in combination with lacquer polyisocyanates, as binders in low-solvent or solvent-free, high solids coating compositions because they provide rapid curing of the coatings at low temperatures.
These secondary polamines are polyaspartates and are described, e.g., in U.S. Pat. Nos. 5,126,170, 5,214,086, 5,236,741, 5,243,012, 5,364,955, 5,412,056, 5,623,045, 5,736,604, 6,183,870, 6,355,829, 6,458,293 and 6,482,333 and published European Patent Application 667,362. In addition, aspartates containing aldimine groups are also known (see U.S. Pat. Nos. 5,489,704, 5,559,204 and 5,847,195). Secondary aspartic acid amide esters are also known (see U.S. Pat. No. 6,005,062). Their use as the only isocyanate-reactive component or mixed with other isocyanate-reactive components in two-component coating compositions are also described in the above-identified patents.
The process for preparing these polyaspartates is the reaction of the corresponding primary polyamines with maleates or fumarates corresponding to the formulaR1OOC—C(R3)=C(R4)—COOR2wherein R1, R2, R3 and R4 are identical or different organic groups, resulting in the formation of secondary polyamines. Due to stearic, structural and electronic effects, these secondary amino groups have sufficiently reduced reactivity towards isocyanate groups to be mixable with polyisocyanates in a reliable and easy manner.
The reaction which is used to prepare polyaspartates is the addition of primary amines to the activated C—C double bond in vinyl carbonyl compounds, which has been described in the literature (see Chem. Ber. 1946, 38,83; Houben Weyl, Meth. d. Org. Chemie, Vol. 11/1, 272 (1957); Usp. Chimil 1969, 38, 1933). It has been found, however, that this reaction does not proceed to completion during the course of the actual synthesis process (e.g., 24 hours with stirring at 60° C.). The actual extent of the reaction is dependent upon the type of primary polyamine. Thus, the degree of conversion (measured by the concentration of free, unconverted maleate and fumarate, into which maleate rearranges in the presence of basic catalysts) after 1 day with 1,6-hexanediamine is about 90 to 93%. The degree of conversion after 1 day with a cycloaliphatic polyamine having sterically hindered primary amino groups, i.e., 4,4′-diamino-3,3′-dimethyldicyclohexylmethane is only 77% Complete or essentially complete conversion is achieved only after several days or, in the case of 4,4′-diamino-3,3′-dimethyldicyclohexyl-methane, only after several months.
In a typical commecial production, the reaction is run for sixteen hours when the conversion is somewhere between 75 and 95% complete depending on the amine used. The “unfinished” material is drummed and held in storage until the reaction is complete. This typically takes anywhere from two weeks to six months. U.S. Pat. No. 5,821,326 describes the use of certain five-membered aromatic ring compounds as catalyst to accelerate the preparation of the aspartates.
The conventional aspartates are capable of a further transformation (after curing with an isocyanate) to form a hydantoin ring structure. This hydantoin formation might lead to a shrinking of the coating and undesired alcohol formation. It would also be desirable to prepare an aspartate that would be less prone to hydantoin formation.